This invention is in the field of circuits for starting and operating fluorescent lamps from low frequency a-c power. In particular, this invention is an improvement over that invention described in the U.S. Pat. No. 4,399,391.
Various circuits have been devised for starting and operating fluorescent lamps, and for heating or preheating their cathodes. U.S. Pat. No. 4,185,233 to Riesland, Hammer and Lemmers discloses a circuit in which cathodes of fluorescent lamps are heated by a transformer, and U.S. Pat. No. 4,207,497 to Capewell et al. discloses a high frequency lamp operating circuit in which the cathodes are heated by a transformer having a primary winding connected in series with a capacitor to the a-c power source, the primary winding and/or ballast inductor in combination with the capacitor, being resonant at or near the frequency of the a-c power source; the transformer is connected to provide constant cathode voltages during the high frequency lamp operation and dimming. U.S. Pat. No. 3,611,021 to Wallace and U.S. Pat. No. 4,207,497 to Capewell also disclose high-frequency circuits for starting and operating fluorescent lamps, and employ a resonant circuit tuned to a single individual harmonic of the high-frequency (20 kilohertz) operating current source to aid in starting the lamps.
Other fluorescent lamp circuits have been devised which turn off the cathode heating power while the lamps are operating. For example, U.S. Pat. Nos. 2,330,312 to Raney, 4,009,412 to Latassa, and 4,146,820 to Bessone disclose circuits having magnetically operated switches which open to disconnect the cathode heating circuit when the lamps are operating; U.S. Pat. Nos. 2,354,421 to Pennybacker, 2,462,335 to Reinhardt, and 4,097,779 to Latassa disclose thermostatic cathode heating disconnect switches; and U.S. Pat. No. 4,010,399 to Bessone discloses solid state switches for the same purpose.
In some cases, the primary coil is subject to undesirable high temperatures if one or both of the lamps should rectify the high temperature sometimes causing damage to the coil and ballast.
In the circuit of U.S. Pat. No. 4,399,391, a single switch 21 is utilized which is preferably a voltage actuated bidirectional diode such as SIDAC. Although the utilization of a single SIDAC in the circuit of the '391 patent did result in satisfactorily operating the circuit, it has been found that it is preferable to stack a plurality of the SIDACS in series.